Intermetallic iron aluminide alloys, such as Fe.sub.3 Al, have been of long-standing interest because of their excellent abrasive wear resistance, corrosion and sulfidation resistance, oxidation resistance and resistance to cavitation erosion. Application of ion aluminides in industry has been hampered by brittle behavior at room temperature and insufficient strength at elevated temperature. However, some advances in alloy development and processing have somewhat improved ductility and elevated temperature strength.
Conventional methods of processing iron aluminides, such as Fe.sub.3 Al, include casting, hot rolling, and powder metallurgical processing. A recently developed alternative processing method is reactive sintering. Here, reactive sintering or self-propagating high temperature synthesis is utilized. The advantages of reactive sintering include inexpensive and easily compacted powder starting materials, low processing temperatures, and flexibility in composition and micro-structure control, including the ability to incorporate particulate reinforcements. The process uses an exothermic reaction between elemental powders to form the intermetallic by the reaction: EQU 3Fe+Al&gt;Fe.sub.3 Al
During heating of elemental powder compacts, compound formation occurs initially by solid state reaction at interparticle contacts. This process causes local heating due to an exothermic reaction and results in localized liquid formation. The presence of the aluminum-rich liquid causes a rapid increase in the reaction rate and the heat evolved causes further liquid formation. The speed of the overall process suggests that melt formation and spreading, accompanied by exothermic heating, controls the reaction rate. Compound formation occurs by precipitation from the liquid as the liquid front advances outward from the original aluminum particle sites.
Combustion synthesis of Fe.sub.3 Al is somewhat difficult: swelling of compacts accompanying reaction synthesis has been reported. Careful selection of the relative particle sizes, green density and heating rate can result in densification compared to the green state. However, the application of pressure is apparently required to achieve full density. It has been found that the typical added elemental Cr does not dissolve into solution during the formation of Cr-enhanced Fe.sub.3 Al by combustion synthesis. A solution treatment of several hours is required to homogenize the material after formation of the compound. If the solution treatment is carried out subsequent to consolidation, Kirkendall pores result at the prior sites of the Cr particles. It is therefore desirable to react the powders and homogenize the material prior to consolidation, or maintain the pressure while holding the material at a temperature well above that necessary to carry out the synthesis reaction to allow dissolution of the Cr.
Example methods of forming iron aluminides are shown in Knibloe, et al., "Microstructure and Mechanical Properties of P/M Fe.sub.3 Al Alloys", Advances in Powder Metallurgy, Vol. 2, pp. 219-231 (1990), Diehm, et al., "Processing and Alloying of Modified Iron Aluminides", Materials & Manufacturing Processes #4(1), pp. 61-72 (1989); Sheasby, J. S., "Powder Metallurgy of Iron-Aluminum", The International Journal of Powder Metallurgy & Powder Technology, Vol. 15, No. 4, pp. 301-305 (1979); Rabin, et al., "Microstructure and Tensile Properties of Fe.sub.3 Al Produced by Combustion Synthesis/Hot Isostatic Pressing", Metallurgical Transactions A, Vol. 23A, pp. 35-40 (1992); and Rabin, et al., "Synthesis of Iron Aluminides from Elemental Powders: Reaction Mechanisms and Densification Behavior", Metallurgical Transactions A, Vol. 22A, pp. 277-286 (1991). These references are hereby incorporated by reference.
As new and improved materials are developed, methods of joining the material to itself and other materials must be developed. Some progress has been made in joining iron aluminide alloys, such as shown in S. A. David, et al., Welding Journal, 68 (9), 372s (1989) and T. Zacharia, et al., Proceedings of the Fifth Annual Conference on Fossil Energy Materials, p. 197, Oak Ridge, Tenn., (May 1991). Iron aluminide alloys may as well find uses beyond those presently contemplated in the prior art.